In many applications involving large solid structures it is desirable to detect internal disturbances. It is additionally valuable to be able to measure the position of such disturbances without further disturbing the integrity of the structure. In mining and construction it is often required to detect faults or disturbances and their positions in earth and rock formations. To detect such faults an electrical element can rigidly be embedded into the solid structure or earth. When a portion of the solid breaks or moves relative to the electrical elements it causes the electrical element to shear or break. Prior methods have used a ladder like arrangement of parallel resistors as taught by U.S. Pat. No. 3,477,019. Such systems lack accuracy because of their use of discrete components. The accuracy of such system is directly proportional to the number of components and cost. Because these devices determine position by measuring resistance between parallel conductors, any shunting resistance such as moisture paths between the conductors results in an error in position determination. My invention overcomes these inherent faults by using capacitance as a measure of position and by using one continuous inexpensive element.
An elongated electrical element is embedded in a solid formation. This element is made of easily shearable materials and has a capacitance that varies as a function of length. The element acts as an end-feed capacitor having accessible leads attached to the conductive surfaces at one end of the elongated element. When a meaningful disturbance occurs in the formation the frangible element breaks in the area of the disturbance effectively severing the element into at least two lengths and reducing the effective capacitance connected to the leads. The position of the break can be determined by measuring the capacitance at the leads and relating it to the length by the previously known function.
It is often required to know the position of a disturbance in a solid formation such as earth, rock, or formed conglomerates. This information is especially valuable in excavations such as are found in mining and construction for example. In such applications boreholes are normally drilled to test or reinforce the strata or solids. In one application of my invention an elongated electrical element is securely embedded generally axially within these boreholes. The electrical element does not interfere with other objects such as for example, structural rods or bolts that may be included within the same borehole. A hard cementitous material such as for example concrete can be used to embed the electrical element within the borehole. If it is desired to detect a very slight earth movement or no additional strengthening of the earth formation is required, a weaker, more brittle material can be used as the embedding grout. The grout used need only be capable of transmitting the movement or force of the disturbance in sufficient amount to shear or break the electrical element. The electrical element is made of frangible materials that are easily severed by the forces present during a detectable disturbance. Firmly grouting the electrical element into the solid and constructing the element from easily shearable material, causes the element to sever at a point corresponding to the location of a disturbance or movement in the solid.
The point at which the electrical element is severed can be calculated by comparing the electrical characteristics before the disturbance with those after. The electrical element is constructed so that the electrical characteristics are of a known function of the physical length of the elongated element. While any known function is sufficient, it will be desirable to use a continuous linear function so as to simplify the calculations. In certain applications it may be desirable to use a non-linear function which better suits the physical parameters of the disturbance; such as designing the element so that the electrical characteristics vary greater per unit length in the area where disturbances are anticipated so that the highest resolution and accuracy will be obtained in that area.
Variation in the capacitance for non-linear elements may be made by varying the distance between electrical conductive surfaces, increasing or decreasing the area of the electrical conductive surface, or using electrical insulating material of varying dielectric constants between the conductive surfaces.
If the elongated element is constructed to have a capacitance which is a linear function of the elongation, such as a parallel plate capacitor; then the position of the disturbance is directly proportional to the capacitance measured at the exposed leads. Additionally, the ends of the capacitor plates opposite the exposed leads can be shorted together to allow a continuity check from the exposed leads. A positive continuity check indicates an unbroken capacitor and no further capacitance measurement need be taken.
Accordingly, one object of this invention is to provide a means to accurately and economically determine the position of a movement within a solid formation.
Another object is to provide for the detection of breaks in the grout material as a function of the elongated frangible element.
Another object is to provide an inexpensive electrical element that can readily be inserted into a borehole and easily fashioned to the exact depth of the borehole.
Additional objects and features of the present invention will become apparent to those skilled in the art as the following description of certain present preferred embodiments thereof proceeds.